1. Technical Field
This invention generally relates to methods and devices for applying solder to workpieces and more particularly to a device and method for high volume Ball Grid Array (BGA) solder sphere alignment and placement.
2. Background
Electronic fabrication processes include the attachment of various electrical components, including discrete components. This may include, but is not limited to: resistors, transistors, diodes, switching arrays and integrated circuits such as memory devices, microprocessors, and the like, together with a variety of other electrical components such as transformers, connectors, heat sinks connected to a substrate such as a printed circuit board that electrically connects and mechanically supports the variety of components.
Soldering is the most common process used for connecting the variety of electrical components to the printed circuit board and fixing them in position. Soldering involves the use of a low melting point metal alloy, usually of a lead-tin type, that when exposed to heat, typically at temperatures around 450.degree. F., (183.degree. C.), melts and flows between adjoining metal surfaces or contacts, joining the adjacent surfaces together.
Most soldering processes include three basic steps: pre-cleaning and deoxidizing, solder reflow or reflow joining, and post-cleaning of residue. Pre-cleaning and deoxidizing are usually accomplished by applying a flux material to remove contaminants and oxides from the surfaces to be soldered. Soldering joins the surfaces to be soldered when the solder is heated beyond its melting point. Oxides, typically with a higher melting point than solder, can form a barrier and prevent wetting of the surfaces to be soldered if they are not removed prior to the solder reflowing.
In electronic substrate fabrication, lands are commonly formed on an exposed surface of the printed circuit board and, during the solder process, receive a coating of solder. The electrically conductive leads of the variety of electrical components are brought into contact with the electrically conductive lands, heat is applied to raise the temperature of the lands and leads, and the solder is then heated to reflow and physically join the lands and pads. When the solder cools, it resolidifies, thereby providing an electrically conductive connection.
Ball Grid Array (BGA) packages and Chip Scale packages (CSPs) are rapidly becoming a favored configuration for fine and ultra-fine pitch technology. The ability to achieve high density interconnects, while providing an extremely reliable assembly process, has increased the industry wide migration to this component configuration. The high cost of these components drives the need to salvage or reclaim BGA and CSP packages during the assembly or rework process.
Until recently, BGA and CSP packages which had re-workable defects such as solder shorts or low volume solder were removed from the printed circuit assembly (PCA) and disposed of. It is therefore desirable to salvage BGA or CSP packages thereby reducing the cost of rework.
Component removal and subsequent printed circuit board (PCB) and BGA and CSP package surface site preparation is important for achieving successful re-work of the assemblies. In order to provide a reliable interconnect site during re-work, one must insure that site preparation be carefully conducted. It is extremely important that parameters such as coplanarity, contamination, pad damage, moisture sensitivity and component handling be strictly controlled.
Component removal from the electronic assembly is typically achieved by employing reflow equipment and must be conducted employing the proper reflow profiles to maintain the integrity of the BGA and CSP packages. The reflow and removal process is one of the most important steps in re-work. Because the removal of the original BGA or CSP package results in the release of excess solder on the substrate from the sphere and printed solder mass, it must be carefully removed to provide a planar contamination-free attachment sight. There are several methods by which this can be completed, the most common of which is the utilization of Solder-Wick.TM. (high purity copper braid).
Another common method of removal is the use of a non-contact scavenging system that utilizes nitrogen gas and a vacuum apparatus to lift the molten solder off the lands without physically touching the substrate lands. These systems do provide a superior removal method by controlling temperature and contact damage to the attachment sites. These systems can be rather expensive and thus are not in consistent use as a result.
Following the removal and scavenging of the BGA and CSP packages, a thorough visual inspection may take place. Component land damage is a common problem when dealing with contact scavenging methods. Poor or damaged masking patterns, reduction of the mask volume or physical damage can cause a multitude of solder-ability problems such as shorting.
Following the removal, scavenging and inspection of reclaimed BGA and CSP packages, the BGA and CSP packages must be re-balled. Re-balling involves the alignment and the placement of the solder spheres on the electrically conductive lands. A variety of methods are currently employed for accomplishing this task.
The first is to hand place solder spheres onto fluxed locations. This is quite obviously a slow process.
A second method involves the use of mechanical equipment to individually place sphere pre-forms onto a fluxed or solder printed BGA or CSP package land. This is achieved using an automated encapsulation or dispensing machine which is designed specifically for the alignment and placement of the packages being assembled and which has been programmed to place the spheres in a pre-selected pattern. This mechanism works much like a pick and place machine utilizing x, y, and z coordinate systems.
This process, however, is labor intensive, due to programming requirements, although the results tend to be highly accurate and repeatable. This automated process involves the placement of the component in an alignment tool on an x-y platform. The dispensing machine then may distribute flux in a pre-established and controlled volume onto each placement site. A separate needle vacuum fixture retrieves a single ball from a sphere pool and places it on the fluxed land. This process is repeated until the array has been completed.
The major benefit to this system is the accuracy and semi-automatic use. The major drawbacks with this system are the cost of initial implementation, programming costs and the cost associated with changing the platform. Because the system is designed around a single package, the shift from a primary package design can lead to substantial expense.
Another method uses a paper array which has pre-formed solder spheres impregnated in the paper matrix which corresponds to the circuit package being reballed. The paper array may be purchased with hundreds of different array designs and is relatively inexpensive and readily available. The process involves the normal component preparation as seen with other re-balling processes. First, a re-balling flux is brushed lightly onto the component. The paper pre-form is then placed on the package in an alignment fixture. The entire system is reflowed to provide attachment. After reflow the paper matrix is dissolved in a de-ionized water bath leaving only the attached balls.
The major benefits to this system are accuracy, speed, and ease of use, however, there are two drawbacks. First is the need to purchase the paper matrix. Secondly the system does not work well on under-molded BGAs due to the fact that the molding increases the standoff height of the paper matrix.